The Experimental Silicification of Bacillus subtilis

Silica, particularly the stable phase of silica, quartz is chemically and physically resistant to weathering. Microorganisms are commonly incorporated into silica deposits formed by silica precipitation from geothermal fluids that are over saturated with respect to silica. Silica deposits have provided the oldest evidence of life identified on Earth in the form of fossil microorganisms. The first signs of life found beyond Earth may occur as microfossils in silica deposits. Experiments were conducted on the gram positive bacterium Bacillus subtilis to test the effects of silica concentration, dissolved Fe, and dissolved Al upon silicification processes. Results from initial experiments demonstrated that Bacillus subtilis a common soil bacterium, persists in better condition in the presence of Fe and that silica interacts with cellular surfaces in the presence and absence of Fe. The initial experiments were conducted by combining a suspension of bacterial cells with stock solutions in a 1/1 ratio. Results show a decline in silica concentration although the initial concentration was below calculated saturation. The pH of experimental solutions also declined during the experiment. These results led to the development of a different experimental design. The final experimental design involved introducing small volumes of bacteria to volumes of experimental solution at a 1/125 ratio. The experimental parameters were expanded to include three concentrations of silica intended to represent concentrations well below saturation, approximately at saturation, and 2.5 X saturation. All experiments were conducted in the presence of iron (Fe) and aluminum (Al) (separately) and with no additives (except silica). pH values declined but reached a state of stability within 7 days. The lowest concentration of silica used in the presence of Fe produced the best-silicified bacteria based on SEM/EDS results. The results demonstrate that bacteria do maintain better cellular fidelity and persist for longer time periods in the presence of Fe under all experimental conditions. Al is shown to be important in facilitating a smooth coating on cells, this may produce silicified microfossils that are more resistant to diagenetic and weathering processes. A combination of Fe and Al in silica-rich solutions may produce robust microfossils that can persist through lithogenesis